As the era of driverless cars looms, a new Boston-based study suggests the vehicles may increase traffic congestion in downtown areas as more people embrace ride-hailing services and abandon transit.The same study also found, however, that driverless vehicles could dramatically improve traffic conditions in the suburbs.The study by the World Economic Forum and the Boston Consulting Group is based on a simple premise, backed by 10 months of research and one of the most specific surveys done on autonomous cars: As the cost of using services such as Uber and Lyft comes down because the companies no longer need to pay drivers, people who take mass transit will opt for door-to-door service rather than the bus or subway."If itís the same price or only slightly more, plus I donít have to walk to the station, I donít have to worry about carrying an umbrella, I donít have to check schedules, I donít have to connect, why would anyone take the train?" said Augustin Wegscheider, one of the reportís authors.Why might that cause 5.5 percent more congestion in downtown Boston? Because raid-hailing fleet vehicles will begin to replace private cars."You might have fewer vehicles on the road, but the vehicles that are on the road are operating 12 hours a day," Wegscheider said. "The average car in Boston is operated for something like 12 minutes a day."The study underscores, he said, "that unless you put the right policies in place to actively manage the adoption of [autonomous vehicles], congestion might actually get worse."The answer there, the study says, is that those policies must encourage people to share autonomous vehicles provided by ride-hailing companies."One of the answers is getting more people in the car," Wegscheider said. "If you ask consumers, ĎWould you do that?í they say, ĎHell, no, Iím not going to share a car with strangers,í but if it costs $2 instead of $7, they say, ĎOkay.í "The situation will be very different in most suburbs, the study found, where ride-hailing services "will mainly replace the use of personal cars rather than mass transit, and travel time will decrease by 12.1 percent.""There, the trade-off is instead of sitting in traffic for a 10-mile trip and dealing with it myself and paying $30 for parking in the city every day, I could invest those thirty dollars in the ride service and use the time productively," Wegscheider said.And the mass transit vs. ride-hailing service that applies in urban centers wonít clog suburban streets."If I take the train for an hour now for $6, even if the Lyft price comes down from $80 to $40 [for the same trip], itís still a trade-off between $40 and $6," he said. "People will still ride the train."The study also found that the advent of autonomous cars has much broader acceptance in congested cities abroad than it does in the United States, where a deeply rooted car culture engenders skepticism about relinquishing the steering wheel.The report surveyed 5,500 drivers in 27 cities around the world, finding that slightly more than half of Americans said they want to ride in a driverless car. In two counties where the number of cars has exploded in recent years - China and India - 75 to 85 percent of people said they would welcome autonomous vehicles.The difference, Wegscheider said, is that the United States has a much more established car culture."Some people would go from walking or mainly riding a bike to an autonomous vehicle," he said, "rather than a person who grew up in the suburbs of Detroit who says, ĎIíve been driving all my life.í "Wegscheider says he is distrustful of many surveys about whether people are ready to embrace driverless cars."You can Google a study that says 80 percent of people are scared of [autonomous vehicles] and you can just as easily find a study that says 80 percent of people are excited about [autonomous vehicles]. I find neither of those mean anything," he said. "We tried to make it, in these very specific situations, ĎYouíre going to work, itís a 10-mile trip, youíre traveling by yourself at 7 a.m., itís raining, you have these eight options at the following costs and the following trip experience, which one do you take?í"